1. Field of the Invention
This invention pertains to a powertrain and method of a kinetic hybrid vehicle, such as a gas and/or electric powered vehicle that includes a flywheel. The powertrain and method may be used to store and use energy of the flywheel device for vehicle propulsion.
2. Description of the Related Art
Improving fuel economy is an important objective in vehicle design, since it enables reduced fuel consumption and reduced emissions. In a conventional vehicle powered by an internal combustion engine, fuel economy is generally inversely related to vehicle performance, as the engine generally cannot be downsized to be run at its optimal efficiency without sacrificing performance. Acceleration performance is seen in how much reserve power the vehicle has to overcome its own inertia and increase its speed. The more reserve power, the more quickly the desired acceleration or speed can be achieved, and the better the performance of the vehicle. Hence for performance considerations, the bigger the engine in conventional vehicles, the more reserve power there is to accelerate the vehicle relatively quickly and overcome inertia. On the other hand, this means that when the vehicle is not accelerating, its engine is operating at a lower load level and lower efficiency state, wasting the maximum efficiency potential of the engine. In addition, much of the vehicle's kinetic energy is dissipated as heat in the brakes when decelerating, reducing the vehicle's potential fuel efficiency.
Hybrid electric vehicles (HEVs), which are equipped with another power source and energy storage, may recover a portion of the vehicle's kinetic energy during deceleration, and can use a downsized engine for increased fuel efficiency. By supplementing the power from a smaller engine with power from a traction motor, HEVs can run the engine at increased efficiency compared to conventional vehicles without sacrificing performance. Although more efficient and environment friendly than some conventional vehicles, these electric hybrids may be difficult to produce without the added costs of a large traction motor, controller, and electrochemical and/or electric storage devices. These costs may result in an increased price to consumers that limits market penetration.
Aside from cost, a main disadvantage of electric hybrids is that they are greatly limited in the fuel economy improvements they can provide. Part of conventional electric hybrids' efficiency limitations comes from the fact that energy is not stored in the same form it is used in. When energy from the engine or the vehicle is stored as electricity, there are multiple conversions from mechanical to electric, from electric to electrochemical, from electrochemical to electric, and from electric to mechanical. There are typically four energy transformations by the time the energy is used, each resulting in a conversion loss. These conversion losses typically comprise above one-third the original amount of energy initially recovered, such as from braking. Another part of conventional electric hybrids' efficiency limitations comes from the inherent characteristics of motor/generators and batteries—namely, their power transfer limitations and reduced efficiency at high rates of charge and discharge. Even when the electric storage consists of ultracapacitors, which are highly efficient at high rates of charge and discharge, the energy regenerated from deceleration is limited by the power of the traction motor. Thus only a small portion of the vehicle's kinetic energy may be recovered via regenerative braking in electric hybrids.
To avoid conversion losses and improve fuel economy, an alternative energy storage device is available: the flywheel. Flywheels have much higher power density and can give and receive much higher power than motor/generators, and since flywheels store energy in the same form that it is to be used in for vehicle propulsion, they are more efficient than electrical energy storage devices used in hybrid electric vehicles if the energy is released via a direct mechanical path. The challenge with flywheels is how to control the amount of energy transferred. Flywheel systems may use Continuously Variable Transmissions (CVTs) to store and release energy. In the early days of flywheel vehicle development and even now in some industrial applications and Uninterruptible Power Supply (UPS) systems, energy is stored into and released from the flywheel via one or more motor/generator(s), traveling a 100 percent electromagnetic path from source to destination; these flywheel systems also suffer multiple energy conversions and limited efficiency due to conversion losses.
U.S. Pat. No. 7,341,534 by Schmidt discloses an electrically variable hybrid transmission and powertrain equipped with a flywheel energy storage device. In this configuration, based on modifying a conventional Internal Combustion Engine (ICE) driveline, the engine is coupled to the final drive through a torque converter, an automatic transmission, and the transmission shaft. The final drive may include a drive shaft, a differential, a set of fixed gears, and wheels, but does not include a transmission. Meanwhile, the flywheel is coupled to the final drive through a three way power split transmission wherein a first input/output port is coupled to the flywheel, a second is coupled to a motor/generator, and the third is coupled to the transmission output shaft. The motor/generator and the gears comprise a CVT between the flywheel and the transmission output shaft so that part of the energy recovered by the flywheel from the wheels is transferred through a purely mechanical path. The placement of the flywheel and its CVT is after the transmission of the engine, so the variator motor/generator in the flywheel's CVT must operate over a wide range and needs two planetary gear sets to perform the right function. Also, two transmissions are required; an automatic transmission for the engine and the three port power split CVT for the flywheel.
Document US2010/0184549 by Sartre, et. al discloses a similar configuration for the same purpose of energy recovery. Unlike Schmidt, the flywheel energy recovery system for Sartre is located between the engine and the engine's transmission. It takes advantage of the engine's transmission so that the energy recovery system is more independent from the vehicle speed than that in U.S. Pat. No. 7,341,534. The variator motor/generator for the flywheel operates over a narrower range than in Schmidt.
In both the configurations of Schmidt and Sartre, the CVT for the flywheel is a three way power split transmission embodied by planetary gears and at least one motor/generator to vary the CVT ratio for the aforementioned power split transmission. Both use three-port power split devices as transmissions only for the flywheel, so the engine needs a separate transmission. Another disadvantage is that both systems may have critical points where the variator motor/generator approaches zero speed (stall state, maximum current) and the system has poor efficiency unless the effect is mitigated through other means such as by mechanically braking the variator port when the motor/generator approaches zero speed.
Both the configurations of Schmidt and Sartre have coupled the engine and the final drive on the same port of the power split transmission. With the engine and the final drive both connected to the same port on the power split transmission, another transmission may be needed between the engine and the final drive to vary the relative speeds of the engine and the final drive. In these configurations, the CVT only serves the flywheel, so the engine needs its own separate transmission.